In order to reveal the impact of the application of biogas slurry instead of chemical nitrogen fertilizer on the environmental risk of heavy metals in the soil by returning straw to the field, four treatments, without biogas slurry and without straw applications (CK), biogas slurry application without straw (B), straw application without biogas slurry (S), and biogas slurry combined with straw applications (BS), were applied in a typical coastal reclaimed farmland (rice-wheat rotation) in Jiangsu province. The migration and morphological characteristics of Cu, Zn, Cd, and Pb in different soil layers were observed, and the potential environmental risks were estimated. The results showed that:① The total amounts of Zn and Pb in the surface soils (0-20 cm) in the rice and wheat fields under the BS treatment decreased significantly (P<0.05). The four heavy metals in the paddy soils migrated 6%-11% from the surface to the middle and lower layers (20-60 cm), and Cu, Cd, and Pb in the wheat soils migrate down from the surface by 25% to 33%. This indicated that the combined use of biogas slurry and straw accelerates the vertical downward movement of heavy metals in the surface soil. ② Under the BS treatment, the contents of the weak acid extraction of Cu in the surface soil of the paddy field decreased by 8.8%, and the residual state of Zn, Cd, and Pb decreased by 7.0% to 14.2%. This revealed that Cu was passivated, but Zn, Cd, and Pb tended to be activated. In comparison, the reduction in Cu residues in wheat field surface soil was 2.8 times that of the weak acid extraction, indicating that Cu was activated. Furthermore, the residue state of Cd increased, the weak acid extraction state of Pb decreased, and Cd and Pb were passivated. ③ The ecological risk assessment of heavy metals showed that there is no ecological risk in the soils under the BS treatment, and the risk indices were significantly lower than those of the B and S treatments (P<0.05). Therefore, the combined use of biogas slurry and straw helps to significantly reduce the risk of heavy metal pollution in the soils in the coastal reclamation areas.
Keywords: biogas slurry; heavy metal; rice-wheat double cropping field; soil; straw.